37 Next-Generation Random Vibration Tests 403 -200 -100 0 0 500 1000 1500 Wind Tunnel CSD 4Z/6Z Twin Shaker CSD 4Z/6Z Frequency (Hz) Acceleration CSD Phase (degrees) 2000 100 200 300 Fig. 37.11 Example of phase plot from twin-shaker test many CSDs from the twin-shaker test; it is not necessary to present them all in this paper and the plots in Fig. 37.14 are representative of the full data set. A noteworthy result is the phase of the CSDs and a representative curve is shown in Fig. 37.11. These plots show that the twin-shaker test was poor at replicating the relative phase between some of the locations when compared to the original aerodynamic environment. This has significant consequences as it means that the operating deflection shapes in the laboratory test, and the associated stress patterns are very different to the aerodynamic environments it is attempting to replicate. 37.6 Impedance Matched Multi-Axis Test (IMMAT) A new approach to vibration testing has been developed and can offer significant enhancements over traditional vibration testing methods. This new approach is called Impedance Matched Multi-Axis Testing (IMMAT) and has the following critical characteristics: (i) The local impedance from the parent structure is included in the vibration test. (ii) The structure is excited in all axes simultaneously. (iii) The attachment of the exciters has minimal influence on the dynamics of the structure. (iv) The vibration test is controlled at many response locations using MIMO control. The IMMAT approach was applied to the missile and is shown in Fig. 37.12. The local impedance from the wind tunnel environment was simulated by including the launcher rail and a section of wood with the same thickness as the wooden ceiling of the wind tunnel. The missile was excited in three orthogonal axes simultaneously using electrodynamic shakers attached via flexible drive rods. The vibration response of the IMMAT was controlled at the thirteen accelerometers using the Leuven Measurement Systems (LMS) MIMO Random Control software and the Supervisory Control and Data Acquisition System (SCADAS) hardware. The test specification consisted of all of the PSD and CSD measurements from the wind tunnel environment and was comprised of thirteen PSDs and 78 CSDs. This resulted in a test specification with approximately 60,000 breakpoints; a breakpoint is a frequency/amplitude data point, a set of which defines the outline of the test specification and typically consists of only tens of breakpoints in traditional vibration tests. The PSDs from the IMMAT are shown in the appendices (Fig. 37.15), with only twelve of the thirteen shown for convenience. A representative selection of jCSDj plots and CSD phase plots are displayed in the appendices (Figs. 37.16 and 37.17). The plots demonstrate that the IMMAT was able to accurately simulate the wind tunnel environment, including the wind tunnel fan harmonics. 37.7 Discussion and Conclusions A case study has been presented which demonstrates some of the limitations with the conventional twin-shaker, single-axis, vibration test for underwing missiles, in particular, the poor simulation of the aerodynamic environment at uncontrolled locations during the vibration test. This was apparent in the severity of the overtest at some locations and for portions of the
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